1. Field of the Invention
Processes and apparatus for separation of waste latex from liquid waste streams in latex paint manufacturing, latex coating of paper, latex adhesive manufacturing, and any other application generating aqueous latex waste. The process is useful for the recovery of the latex solids and the clarified filtrate for reuse, recovery or more efficient disposal thereof.
2. Description of Related Art
Latex paints, coatings and adhesives are used in a wide variety of residential, commercial, and industrial applications in which protective and/or decorative coatings are needed on interior and exterior structures as well as on papers and paper or film products. Recent federal and state environmental regulations have mandated reductions in the volatile organic content (VOC) of paints and other coatings. The use of latex paints and coatings is considered to be advantageous because the primary vehicle in a latex paint is water. The VOC of a latex paint is minimal compared to an “oil based” paint, in which the primary vehicle consists mainly of aliphatic and/or aromatic hydrocarbons.
Although the use of latex paint is advantageous from an environmental and health standpoint, the manufacture of latex paint and other latex emulsions does have problems to be addressed. In latex paint, coating, and adhesive manufacturing, a variety of liquid wastes containing emulsified latex are produced. The liquid waste may contain latex at a range of concentrations.
At certain points in the manufacturing process, in which the latex is highly concentrated as “liquid latex”, a batch may be considered “out of spec” with respect to quality metrics, and must be discarded. Liquid latex may be between about 50 to about 70 weight percent solids of latex. At other points in the manufacturing process, manufacturing errors or batch “ends” may occur, resulting in the production of material generally referred to as “latex waste solution.” This latex waste solution may be between about 30 to about 40 weight percent solids of latex. Finally, during the cleaning of vessel, piping, and other material handling equipment, waste liquid is produced as “latex wash solution.” This liquid waste may be between about 3 to about 20 weight percent solids of latex.
These waste streams may be mixed together within the manufacturing plant to produce a single batch or liquid latex waste, or they may be maintained as separate wastes. In either case, a significant volume of liquid latex waste is produced in a latex paint, latex coating or adhesive manufacturing operation or a latex industrial use. Although latex waste is considered to be “environmentally friendly” and of low toxicity compared to oil-based paint waste, the discharge of such large volumes of latex waste to public sewers and natural waterways is prohibited by law.
Concentration of the latex by evaporation is not economically feasible. The energy cost to evaporate the water from the waste is too high, and following evaporative concentration, the remaining material left behind is a highly viscous sludge that is difficult to handle, and may still unacceptable for disposal in a landfill.
Processes and chemical treatment regimens for flocculation of latex paint waste streams are known. For example, U.S. Pat. No. 4,401,574, “Flocculation of Aqueous Paint Waste,” of Farrington et al. discloses a process in which paint waste water containing dispersed paint solids is clarified by flocculating the solids with a basic poly aluminum chloride having the structural formula Aln(OH)mCl3n-m, wherein n and m are integers and 3n is larger than m, and the basicity is from 30% to 83%. The disclosure of this United States patent is incorporated herein by reference.
In this patent, there is no detailed disclosure of a suitable process for filtration and dewatering of the floc resulting from the flocculation process; only the general statement, “If desired, the water content of the settled floc (sludge) can be further reduced by filtration or centrifugation.” The floc that results from the process of Farrington et al. can be particularly difficult to handle in filtration and dewatering processes. The selection of a suitable filter and suitable operating conditions for satisfactory separation and dewatering of latex flocs has remained a problem in the industry.
What is needed is a low-cost process for treating liquid latex waste which can accept incoming waste at a range of concentrations, convert the latex to a solid phase, separate the solid phase from the liquid phase, dewater the solid phase to provide a solid material suitable for landfill disposal or recycling into another product or process stream, and render the liquid phase suitable for the industrial waste treatment process, or suitable for discharge to public sewers and/or waterways, or potentially recycling back into the manufacturing process with or without additional treatments.